Bias in Sunscreen SPF Testing: A Review of Published Data.

Abstract

Subversion bias, a type of selection bias, through manipulation of subject recruitment compromises data validity. This study explores the possibility of subversion bias in sunscreen sun protection factor (SPF) testing. It has been established that subjects with lower minimal erythemal dose (MED) values exhibit higher sunscreen SPF values. Consistency of this response is determined in subjects who participated in multiple sunscreen efficacy clinical trials. All trials determined the SPF of the sunscreen standard P2. Of the 652 subjects with greater than three observations (n = 286), 35 subjects consistently had values either well above (n = 29) or below (n = 6) the average SPF value of the dataset (15.6 ± 1.2). The difference between the average SPF by the subject exhibiting the highest average SPF for P2, 19.8 ± 0.9, and the subject exhibiting the lowest average SPF for P2, 12.3 ± 2.6, is 7.5 SPF units, or 61%. Recruitment strategies based on historical SPF values for an individual would be considered subversion bias. Foreknowledge of those subjects with consistent results either in favor or not in favor of SPF testing outcomes could be exploited and would provide a reason for variation in results among testing facilities.